What determines the structure of short gamma-ray burst jets?

Abstract

The discovery of GRB 170817A, the first unambiguous off-axis short gamma-ray burst arising from a neutron star merger, has challenged our understanding of the angular structure of relativistic jets. Studies of the jet propagation usually assume that the jet is ejected from the central engine with a top-hat structure and its final structure, which determines the observed light curve and spectra, is primarily regulated by the interaction with the nearby environment. However, jets are expected to be produced with a structure that is more complex than a simple top-hat, as shown by global accretion simulations. We present numerical simulations of short GRBs launched with a wide range of initial structures, durations and luminosities. We follow the jet interaction with the merger remnant wind and compute its final structure at distances ≳ 1011 cm from the central engine. We show that the final jet structure, as well as the resulting afterglow emission, depend strongly on the initial structure of the jet, its luminosity and duration. While the initial structure at the jet is preserved for long-lasting SGRBs, it is strongly modified for jets barely making their way through the wind. This illustrates the importance of combining the results of global simulations with propagation studies in order to better predict the expected afterglow signatures from neutron star mergers. Structured jets provide a reasonable description of the GRB 170817A afterglow emission with an off-axis angle θobs ≈ 22.5○.

Linking the rates of neutron star binaries and short gamma-ray bursts

“…Typically, BNS mergers have more ejecta than NSBH mergers along the poles [e.g., 40,67], which is the region where jets are launched. Numerical simulations already show that propagation through ejecta helps collimate gamma-ray burst jets [7,68]. We therefore believe that gamma-ray burst jets launched in NSBH mergers are likely to be less collimated than ones launched in BNS mergers.…”

“…As described in Sec. II, interaction with the ejecta around the polar region helps collimate jets [7,68]. We do not expect every BNS merger or NSBH merger to eject the same amount of matter so one cannot reasonably expect there to be a universal opening angle θ j across multiple BNS mergers, let alone between BNS and NSBH mergers.…”

Short gamma-ray burst jet propagation in binary neutron star merger environments

“…Such an effort, strongly boosted by the observation of GRB 170817A, includes semi-analytical models (e.g., Salafia et al 2020;Lazzati et al 2020;Hamidani & Ioka 2021 and refs. therein) as well as two-or threedimensional (magneto)hydrodynamic simulations in the framework of special or general relativity (e.g., Nagakura et al 2014; Lazzati et al 2018;Xie et al 2018;Kathirgamaraju et al 2019;Geng et al 2019;Nathanail et al 2020;Murguia-Berthier et al 2021;Urrutia et al 2021;Nathanail et al 2021;Gottlieb et al 2021 and refs. therein).…”

Jet propagation in expanding media

“…The propagation of relativistic jets in expanding medium started to draw special attention in anticipation of the first GW signal from binary neutron star (BNS) mergers (Murguia-Berthier et al 2014Lazzati et al 2017;Gottlieb et al 2018a), and it became the focus of many studies following GW170817 (Kasliwal et al 2017;Gottlieb et al 2018bGottlieb et al , 2021aDuffell et al 2018;Kathirgamaraju et al 2018Kathirgamaraju et al , 2019Lazzati & Perna 2019;Geng et al 2019;Gottlieb & Loeb 2020;Klion et al 2021;Murguia-Berthier et al 2021;Urrutia et al 2021). Most of these studies used numerical simulations showing that the jet-ejecta interplay shapes the emerging jet-cocoon structure, and thus has important observational implications on the prompt emission and the afterglow light curve, and possibly also on the kilonova emission.…”